1. Field of the Invention
This invention relates to the manufacture of an optical fiber preform. More specifically, the invention relates to a method of manufacturing an optical fiber preform having a pattern of voids extending longitudinally through the preform.
2. Background of the Invention
A photonic crystal is a structure having a periodic variation in dielectric constant. The periodic structure may be 1, 2 or 3 dimensional. The photonic crystal allows passage of certain light wavelengths and prevents passage of certain other light wavelengths. Thus, the photonic crystals are said to have allowed light wavelength bands and band gaps that define the wavelength bands that are excluded from the crystal.
At present, the wavelengths of interest for telecommunication applications are in the range of about 800 nm to 1625 nm. Of particular interest is the wavelength band in the range of about 1300 nm to 1600 nm.
Light having a wavelength in the band gap may not pass through the photonic crystal. However, light having a wavelength in bands above and below the band gap may propagate through the crystal. A photonic crystal exhibits a set of band gaps which are analogous to the solutions of the Bragg scattering equation. The band gaps are determined by the pattern and period of the variation in dielectric constant. Thus the periodic array of variation in dielectric constant acts as a Bragg scatterer of light of certain wavelengths in analogy with the Bragg scattering of x-rays wavelengths by atoms in a lattice.
Introducing defects into the periodic variation of the photonic crystal dielectric constant can alter allowed or non-allowed light wavelengths that can propagate in the crystal. Light which cannot propagate in the photonic crystal but can propagate in the defect region will be trapped in the defect region. Thus, a line defect within the crystal can serve as a localized “light tunnel”. Specifically, a line defect in the photonic crystal can act as a waveguide for a mode having a wavelength in the band gap, the crystal lattice serving to confine the guided light to the defect line in the crystal. A particular line defect in a three dimensional photonic crystal would act as a waveguide channel for light wavelengths in the band gap. A review of the structure and function of photonic crystals is found in, “Photonic Crystals: putting a new twist on light”, Nature, vol. 386, Mar. 13, 1997, pp. 143-149, Joannopoulos et al.
A first order band gap phenomenon is observed when the period of the variation in dielectric constant is of the order of the light wavelength which is to undergo Bragg scattering. Thus, for the wavelengths of interest, i.e., in the range of about 1300 nm to 1600 nm, as set forth above, a first order band gap is achieved when the period of the variation is about 500 nm. However, photonic crystal effects can occur in crystals having dielectric periodicity in the range of about 0.1 μm to 5 μm. Nevertheless, a two or three-dimensional photonic crystal having even a 5 μm spatial periodicity is difficult to fabricate.
Conventional processes have been used to create glass honeycomb structures suitable for forming photonic crystals. The prior art approaches to manufacturing this type of glass honeycomb article are either to fuse individual rods and/or hollow tubes together or to machine out a solid piece of glass to form a multi-channeled article.
These prior art processes are problematic for several reasons. Firstly, it is difficult to fuse multiple rods and/or hollow capillary tubes to form a multi-channeled article which can then optionally be hot-drawn down and re-bundled again and again into a progressively finer and finer array of hollow channels. Secondly, it is difficult to assemble and fuse multiple rods and/or hollow tubes uniformly into a perfect honeycomb structure. Thirdly, the diameter of the individual rods and/or hollow tubes that can be easily handled limits the number of tubes in the first bundle towards making the honeycomb structure, because there is a practical limit to the diameter of the assembly that can be uniformly hot-drawn down. Lastly, it is extremely expensive and time consuming to machine a multitude of deep channels into a glass object.
Ceramic honeycomb structures such as Celcor® (a cordierite honeycomb structure used commercially as a substrate for automotive catalytic converters. Celcor® is a registered trademark of Corning Incorporated) and glass-ceramic mixtures have been paste-extruded from particulate material, but the resulting honeycomb article is not transparent to light, significantly reducing its utility. In addition, the honeycomb article is crystalline in nature, making post-forming operations difficult. Further, the particle sizes of the raw material used in the Celcor® process are relatively large. The particle size can significantly affect the minimum wall thickness for an extruded honeycomb structure.
U.S. Pat. No. 6,260,388 to Borrelli discloses a method of making multi-channeled structures by extruding a silica-containing paste. In this method, a paste comprised of glass soot powders and a binding agent is extruded and sintered to form an optical fiber preform.
A disadvantage of the foregoing extrusion method is the low porosity and poor permeability of the greenware body as a consequence of high-pressure extrusion. Although light propagating in a photonic crystal structure comprising a plurality of channels or passageways propagates principally in a central channel or channels, a small percentage of the light also propagates in the walls of the channels. Contaminants contained in the channel walls therefore contribute to loss, or attenuation, of the propagating light.
Thus, the deficiencies inherent in prior art processes leave open the need for a method of manufacturing a photonic crystal optical fiber preform that is straight-forward to implement, and flexible enough to provide an optical fiber preforms with a variety of channel patterns.